npm test ERROR in multi, related to font awesome scss

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**

Executing `npm run test` produces an error that stops tests from being run.

```
ERROR in multi ./node_modules/bootstrap/dist/css/bootstrap.css ./node_modules/typeface-exo/index.css ./node_modules/roboto-fontface/css/roboto/roboto-fontface.css ./node_modules/ionicons/scss/ionicons.scss ./node_modules/font-awesome/scss/font-awesome.scss ./node_modules/socicon/css/socicon.css ./node_modules/nebular-icons/scss/nebular-icons.scss ./node_modules/pace-js/templates/pace-theme-flash.tmpl.css ./src/app/@theme/styles/styles.scss
Module not found: Error: Can't resolve 'C:\Users\Gabriel\Documents\Github\dizco\ngx-admin\node_modules\font-awesome\scss\font-awesome.scss' in 'C:\Users\Gabriel\Documents\Github\dizco\ngx-admin'
@ multi ./node_modules/bootstrap/dist/css/bootstrap.css ./node_modules/typeface-exo/index.css ./node_modules/roboto-fontface/css/roboto/roboto-fontface.css ./node_modules/ionicons/scss/ionicons.scss ./node_modules/font-awesome/scss/font-awesome.scss ./node_modules/socicon/css/socicon.css ./node_modules/nebular-icons/scss/nebular-icons.scss ./node_modules/pace-js/templates/pace-theme-flash.tmpl.css ./src/app/@theme/styles/styles.scss styles[4]
```

**Expected behavior:**

Tests should be executed normally, without errors.

**Steps to reproduce:**

1. Clone ngx-admin
2. `npm install`
3. `npm test`

**Related code:**

This has likely been introduced by #1798

### Other information:

**npm, node, OS, Browser**
npm 6.4.1

**Angular, Nebular**
ngx-admin 3.0.0